Reading from a tape storage medium

ABSTRACT

In a method for operating a tape storage system, readback signals are simultaneously received from a set of data readers reading from data tracks of a set of adjacent data tracks extending along a longitudinal extension of a tape storage medium, wherein each data track of the set has a width and each data reader of the set has a width equal to or less than the width of a data track. Simultaneously to receiving the readback signals from the set of data readers readback signals are received from a servo reader arrangement containing at least one servo reader arranged laterally offset from one of the data readers by less than the width of a data track. At least one of lateral position, velocity, gain and timing information is determined dependent on the readback signals of each servo reader in the servo reader arrangement.

FOREIGN PRIORITY

This application claims priority to Great Britain Patent Application No.1317264.8, filed Sep. 30, 2013, and all the benefits accruing therefromunder 35 U.S.C. §119, the contents of which in its entirety are hereinincorporated by reference.

BACKGROUND

The present invention relates to a method for operating a tape storagesystem, a tape head assembly and a tape storage system.

A conventional tape storage medium typically comprises servo informationprerecorded in dedicated servo bands that extend next to data bands forstoring data, which servo bands and data bands extend in a longitudinaldirection along the tape medium. For example, according to the LTO(Linear Tape Open) Ultrium specification four data bands are straddledby five servo bands. For reading and/or writing data from/to a data banda tape head assembly is positioned to span the entire width of the databand and at least parts of the two servo bands adjacent to the databand. The tape head assembly comprises a set of data readers andwriters, e.g., sixteen or thirty-two, arranged next to each other alonga lateral extension of the tape head assembly orthogonal to thelongitudinal extension of the tape storage medium when being inserted ina tape drive containing the tape head assembly. The tape head assemblyfurther comprises at least two servo readers at the edges of the set ofdata readers and writers, so that the data readers and data writers arelocated between the servo readers. The servo readers are provided forreading servo information from two adjacent servo bands. The servoinformation in the servo bands typically allows for determining severalessential servo parameters, e.g., a lateral position of the tape storagemedium with respect to the tape head assembly, a tape velocity, and alongitudinal tape position.

For writing data to the data band the tape head assembly is set to adesired lateral position on the tape storage medium and data is writtensimultaneously by the data writers of the set while the tape storagemedium is moved across the tape head assembly in a first direction,e.g., in a forward motion direction. When the tape head assembly hasreached the end of the tape storage medium, the tape head assembly isset to a different lateral position, i.e., laterally offset to theprevious position, the tape motion direction is reversed and data iscontinued to be written simultaneously by the data writers of the setwhile the tape storage medium is moved across the tape head assembly inthe reverse motion direction. Hence, the data band is recorded in aserpentine fashion.

A data track is referred to as a longitudinal track within a data bandwritten by a single data writer. In serpentine recording, a set of datatracks written at the same time is referred to as a wrap. In serpentinerecording, a set of data tracks written by the same data writer withinthe data band is also referred to as a data sub-band. The number of datatracks contained in a data sub-band, which also represents the number ofwraps in the data band, depends on a width of the data band, a width ofthe data writers and a resolution of lateral position information thatis enabled by the servo patterns in the servo band. In order to enablewriting in the serpentine fashion, the data writers are arranged offsetfrom each other on the tape head assembly along its lateral extension. Adistance between two adjacent data writers may be set to the number ofwraps to be recorded in the data band times the width of a data tracksuch that data tracks written in the same wrap are offset from eachother for leaving a gap in between for writing data tracks in subsequentwraps into the gap.

The servo information prerecorded in the servo bands allows foridentifying a lateral position of the tape storage medium with respectto the tape head assembly when being read by the dedicated servoreaders. Based on the identified lateral position the tape head assemblymay also be set to a new desired lateral position at the end of a wrapfor writing a new wrap in a reverse motion direction of the tape storagemedium. In addition, the prerecorded servo information allows fordetermining a lateral deviation of the tape storage medium from adesired lateral position during writing and/or reading data. For thispurpose, a position error signal representing the lateral deviation maybe translated into a control signal for an actuator for controlling alateral position of the tape head assembly with respect to the tapestorage medium.

In an alternate way of writing data to a tape storage medium, which isalso referred to as adjacent track recording, the tape head assemblybeing a part of a tape head and containing a transducer array no longerspans the entire width of a data band as defined above but only spans aportion of the width of the data band. Again, the tape head assembly maycontain a set of data readers and writers which are now arranged next toeach other along a lateral extension of the tape head assemblyorthogonal to the longitudinal extension of the tape storage medium whenbeing inserted in a tape drive containing the tape head assembly. Whilein serpentine recording the data writers of the set are arrangedlaterally offset from each other the data writers are now preferablyarranged close to each other without a significant gap in between suchthat data tracks written simultaneously by the data writers are arrangedadjacent to each other in the data band. Given that no additional datatracks are to be recorded in between two adjacent data tracks there isno need for a significant gap in between. Hence, data written byadjacent data writers result in adjacent data tracks on the data bandwhile in serpentine recording data tracks written by adjacent datawriters in one pass result in data tracks gapped from each other. In thelatter case, the gap is filled or respectively will be filled by datatracks written or to be written in different wraps.

In adjacent track recording the set of data tracks simultaneouslywritten next to each other during the tape storage medium moving acrossthe tape head assembly is also referred to as a data sub-band. Thenumber of data sub-bands within a given width of the tape depends on thewidth of the tape head assembly. Once a data sub-band is written and thetape head assembly has reached the end of the tape storage medium, thetape head assembly is set to a new lateral position with respect to thetape storage medium for writing another data sub-band in a reverse tapemotion direction. The offset of the new lateral position to the previouslateral position is at least the width of the tape head assembly toavoid overwriting of the previously written data sub-band.

An advantage offered by adjacent track recording is given by the reducedwidth of the tape head assembly compared to the width of the tape headassembly in the serpentine recording. By having reduced the total spanof the tape head assembly and making a channel pitch, i.e., a pitchbetween adjacent data readers or writers, equal to the track pitch,environmental effects known as tape dimensional stability effects can bereduced. For example, the tape storage medium may laterally expand orcontract dependent on variations in temperature and/or in humidity. Suchexpansion or contraction may more adversely impact the writing andreading of data the larger the tape head assembly span is.

SUMMARY

In one embodiment, a method for operating a tape storage system includessimultaneously receiving readback signals from a set of data readersreading from data tracks of a set of adjacent data tracks extendingalong a longitudinal extension of a tape storage medium wherein eachdata track of the set has a width and each data reader of the set has awidth equal to or less than the width of a data track; simultaneouslywith receiving the readback signals from the set of data readers,receiving readback signals from a servo reader arrangement containing atleast one servo reader arranged laterally offset from one of the datareaders by less than the width of a data track; and determining at leastone of lateral position, velocity, gain and timing information dependenton the readback signals of each servo reader in the servo readerarrangement.

In another embodiment, a non-transitory, computer readable storagemedium having computer readable instructions stored thereon that, whenexecuted by a computer, implement a method for operating a tape storagesystem. The method includes simultaneously receiving readback signalsfrom a set of data readers reading from data tracks of a set of adjacentdata tracks extending along a longitudinal extension of a tape storagemedium wherein each data track of the set has a width and each datareader of the set has a width equal to or less than the width of a datatrack; simultaneously with receiving the readback signals from the setof data readers, receiving readback signals from a servo readerarrangement containing at least one servo reader arranged laterallyoffset from one of the data readers by less than the width of a datatrack; and determining at least one of lateral position, velocity, gainand timing information dependent on the readback signals of each servoreader in the servo reader arrangement.

In another embodiment, a tape head assembly includes a set of datareaders configured to simultaneously read data from a set of adjacentdata tracks of a tape storage medium, each data track having a width;wherein the width of each data reader of the set along a lateralextension of the tape head assembly is equal to or less than the widthof a data track wherein the lateral extension of the tape head assemblyis orthogonal to a longitudinal extension of the tape storage mediumwhen arranged in a tape drive containing the tape head assembly spanningat least the set of data tracks during reading; and a servo readerarrangement containing at least one servo reader arranged laterallyoffset from one of the data readers by less than the width of a datatrack.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention and its embodiments will be more fully appreciated byreference to the following detailed description in accordance withembodiments of the present invention when taken in conjunction with theaccompanying drawings.

FIG. 1 is a top view of a schematic tape storage system according to anembodiment of the present invention;

FIG. 2 is a top view of a schematic tape storage system according to anembodiment of the present invention;

FIG. 3 illustrates top views of a schematic tape storage systemaccording to an embodiment of the present invention, and relateddiagrams;

FIG. 4 is a flowchart representing a method for operating a tape storagesystem according to an embodiment of the present invention; and

FIG. 5 is a block diagram of selected items of a tape storage systemaccording to an embodiment of the present invention.

DETAILED DESCRIPTION

According to an embodiment of a first aspect of the present invention, amethod for operating a tape storage system is provided in which readbacksignals from a set of data readers are simultaneously received whichdata readers read from data tracks of a set of adjacent data tracksextending along a longitudinal extension of a tape storage mediumwherein each data track of the set has a width and each data reader ofthe set has a width equal to or less than the width of a data track.Simultaneously to receiving the readback signals from the set of datareaders readback signals are received from a servo reader arrangementcontaining at least one servo reader arranged laterally offset from oneof the data readers by less than the width of a data track. At least oneof lateral position, velocity, gain and timing information is determineddependent on the readback signals of each servo reader in the servoreader arrangement.

In embodiments, the method may comprise one or more of the followingfeatures: the at least one of lateral position, velocity, gain andtiming information is determined dependent on the readback signals ofeach servo reader in the servo reader arrangement and dependent on thereadback signal of the one data reader; receiving readback signals froma number of servo readers in the servo reader arrangement that is lessthan half the number of the data readers in the set; receiving thereadback signals from the servo reader arrangement containing two servoreaders a first one of which is laterally offset the one data reader bya positive offset value and a second one of which is laterally offsetfrom the one data reader by a negative offset value, and determining theat least one of lateral position, velocity, gain and timing informationdependent on the readback signals of the two servo readers andoptionally further the readback signal of the one data reader;determining the at least one of lateral position, velocity, gain andtiming information by correlating the readback signals of the two servoreaders with the readback signal of the one data reader; simultaneouslyto receiving the readback signals from the set of data readers receivingreadback signals from another servo reader arrangement assigned toanother one of the data readers, the other servo reader arrangementcontaining at least one servo reader being laterally offset from theother one data reader by less than the width of a data track, anddetermining at least one of lateral position, velocity, gain and timinginformation dependent on the readback signals of each servo reader inthe other servo reader arrangement; receiving the readback signals fromthe other servo reader arrangement containing two servo readers a firstone of which is laterally offset by a positive offset value and a secondone of which is laterally offset by a negative offset value, anddetermining at least one of lateral position, velocity, gain and timinginformation dependent on the readback signals of the two servo readersin the other servo reader arrangement and the readback signal of theother one data reader; determining at least one of lateral position,velocity, gain and timing information by correlating the readbacksignals of the two servo readers of the other servo arrangement with thereadback signal of the other one data reader; the data tracks of the setare written simultaneously by means of a set of data writers, the numberof data writers in the set corresponds to the number of data tracks inthe set, and the data writers of the set are arranged offset from eachother with respect to the longitudinal extension of the tape storagemedium; writing to a data track associated with the one data readerincludes one of: exclusively writing data; exclusively writing servopatterns; alternate writing data sections and servo sections each servosection containing one or more defined servo patterns.

According to an embodiment of another aspect of the present invention, atape head assembly is provided for reading data from a set of adjacentdata tracks of a tape storage medium, each data track having a width.The tape head assembly comprises a set of data readers forsimultaneously reading data from the data tracks of the set. The widthof each data reader of the set along a lateral extension of the tapehead assembly is equal to or less than the width of a data track whereinthe lateral extension of the tape head assembly is orthogonal to alongitudinal extension of the tape storage medium when being arranged ina tape drive containing the tape head assembly spanning at least the setof data tracks during reading. The tape head assembly comprises a servoreader arrangement containing at least one servo reader arrangedlaterally offset from one of the data readers by less than the width ofa data track.

In embodiments, the tape head assembly may comprise one or more of thefollowing features: each servo reader of the servo reader arrangementhas a width equal to the width of a data reader; the number of servoreaders is less than half the number of the data readers in the set;each data reader has a width wDR=d*wDT, with wDT being the width of adata track and 0.8≦d≦1; each servo reader of the servo readerarrangement is offset from the one data reader by an offset=f*wDT, withwDT being the width of a data track and 0.3≦|f|≦0.7; the servo readerarrangement contains two servo readers wherein a first one of the twoservo readers is laterally offset by a positive offset value and asecond one of the two servo readers is laterally offset by a negativeoffset value; each servo reader of the servo reader arrangement isoffset from the one data reader by an offset=f*wDT, with wDT being thewidth of a data track and |f|=0.5; each servo reader of the servo readerarrangement has a width less than the width of a data track; the servoreader arrangement contains a single servo reader which is offset fromthe one data reader by an offset=f*wDT, with wDT being the width of adata track and 0.5≦|f|≦0.7, and whose servo reader width is g*wDT, with1.0≦|g|≦1.5; another servo reader arrangement containing at least oneservo reader; the other servo reader arrangement contains a number ofservo readers less than half the number of the data readers in the set;the other servo reader arrangement contains two servo readers arrangedlaterally offset from another one of the data readers by less than thewidth of a data track; the data readers of the set are arranged in atleast one row in the tape head assembly; the one or more servo readersare arranged in at least one different row in the tape head assembly ata different longitudinal position than the at least one row containingthe data readers.

According to an embodiment of another aspect of the present invention, atape storage system is provided comprising a tape head assemblyaccording to any one of the previous embodiments, and a tape storagemedium. The tape storage medium comprises at least one set of datatracks along a longitudinal extension of the tape storage medium andadjacent to each other. At least one data track of the set comprises oneof servo sections or data sections alternating with servo sections.

According to an embodiment of a further aspect of the present invention,a computer program product is provided comprising a computer readablemedium having computer readable program code embodied therewith, thecomputer readable program code comprising computer readable program codeconfigured to perform a method according to any of the previousembodiments.

Embodiments described in relation to the aspect of write head shall alsobe considered as embodiments disclosed in connection with any of theother categories such as the method.

A tape storage medium for storing data (also abbreviated as tape), andin particular a magnetic tape storage medium may be arranged in acartridge which cartridge may be inserted into a tape drive for readingdata from the tape storage medium and writing data to the storage tapemedium. The tape storage medium has a longitudinal extension and a widthorthogonal to its longitudinal extension. The tape is wound onto one ormore reels which are driven by the tape drive in order to make the tapepass by data readers and writers of a tape head assembly for readingand/or writing data from and/or to the tape storage medium.

In connection with embodiments of the invention, a set of adjacent datatracks is written to the tape storage medium preferably in one pass,i.e., simultaneously by means of a tape head assembly comprising anumber of data writers that corresponds to the number of data tracks inthe set. The set of tracks written in one pass is also denoted as datasub-band. Adjacent in this context means that the data tracks written atthe same time are arranged next to each other and remain in thisadjacent arrangement even after additional data sub-bands will have beenwritten by the tape head assembly. The adjacent data tracks in asub-band preferably do not show a significant gap in between. Any gapwidth is assumed to be smaller than a track width. Preferably, assumingthat the data writers are placed in a staggered fashion in the headassembly and assuming that the data writer width is larger than thepitch between adjacent writers, each track written by a data writer ispartially overwritten by the track written by the adjacent data writer,i.e., shingled writing is applied. In contrast, in serpentine recordingdata tracks written in one pass by data writers of a tape head assemblyresult in data tracks gapped from each other which gap is going to befilled by data tracks written in different wraps. For example, in casethe desired width of a data track is 2 μm and 30 data tracks are writtenin one pass in adjacent track recording, the width of the entire set ofdata within a sub-band written by the data writers in one pass may be assmall as 2 μm×30. In contrast, in serpentine recording, the lateraldistance between two data readers in the tape head assembly may e.g., bein the range of 80 μm in order to reserve enough space for other datatracks written in between in future passes of the tape head assembly.

In adjacent track recording, the number of data sub-bands within givenwidth of the tape depends on the width of the tape head assembly. Once adata sub-band is written and the tape head assembly has reached the endof the tape storage medium, the tape head assembly is set to a newlateral position with respect to the tape storage medium for writinganother data sub-band in a reverse tape motion direction. The newlateral position of the tape head assembly with respect to its previouslateral position is at least the width of the set of tracks to avoidoverwriting of the previously written data sub-band is not desired to beoverwritten.

As a result of the foregoing, in a tape head assembly according to anembodiment of the present invention, the data writers are arrangedoffset from each other in the lateral dimension by less than twice thedata track width, and most preferably by the data track width. The widthof a data writer is slightly larger than the track width, e.g., in therange [1 . . . 1.2] times the track width, to allow shingled recording.Lateral in this context denotes in the dimension orthogonal to thelongitudinal extension of the tape storage medium when arrangedunderneath the tape head assembly for reading and/or writing datatherefrom/thereto. In view of the close proximity between adjacent datawriters, crosstalk may occur during data writing. Hence, adjacent datawriters are arranged offset from each other in the longitudinaldimension of the tape head assembly for reducing crosstalk duringwriting. Such arrangement is also referred to as staggered arrangement.

Due to adjacent track recording, it is desirable to also read theconcurrently written set of adjacent data tracks in one pass, i.e.,simultaneously by means of a set of data readers of the tape headassembly. The number of the data readers in the set preferablycorresponds to the number of data tracks.

Therefore, according to an aspect of the present invention, a tape headassembly is provided for reading data from a set of adjacent data tracksof a tape storage medium. The tape head assembly contains a set of datareaders for simultaneously reading data from the data tracks of the set.The width of each data reader is equal to or less than the width of adata track of the set. It is assumed that the data tracks of the seteach have a width orthogonal to a longitudinal extension of the tapestorage medium, and preferably all the data tracks of the set have thesame width such that each data reader of the set has a width along alateral extension of the tape head assembly less than the width of eachdata track. The lateral extension of the tape head assembly is definedcorrespondingly as orthogonal to the longitudinal extension of the tapestorage medium when arranged in a tape drive containing the tape headassembly which then spans at least the set of data tracks duringreading. Preferably, each data reader of the set has a width wDR=d*wDT,with wDT being the width of a data track and 0.8≦d≦1. Preferably, eachdata reader of the set has the same width. It is further desirable thatcenters of the data readers are laterally offset from each other in thetape head assembly by the width of the data track. The center is definedas the geometrical center of a data reader in the lateral dimension. Itis also desirable that the data readers of the set are arranged in a rowin the tape head assembly at a common longitudinal position.

The tape head assembly further comprises a servo reader arrangementcontaining at least one, and preferably two servo readers arrangedlaterally offset from one of the data readers by less than the width ofa data track. Each servo reader preferably has a width equal to or lessthan the width of a data track, and preferably has a width wSR=d*wDT,with wDT being the width of a data track and 0.8≦d≦1. Preferably, eachservo reader of the servo reader arrangement has the same width, andpreferably the servo readers have the same width as the data readers.Each servo reader of the servo reader arrangement shows a defined offsetwith respect to the same one data reader which offset preferably isoffset=f*wDT, with wDT being the width of a data track and 0.3≦|f|≦0.5.In one embodiment the lateral offset is half of the width of a datatrack for each servo reader. In an embodiment with only one servo readerin the servo reader arrangement, the servo reader has an offset=f*wDTfrom the one data reader, with wDT being the width of a data track and0.5≦|f|≦0.7, and servo reader width of g*wDT, with 1.0≦|g|≦1.5.

It is noted that the one or more servo readers are referred to as servoreaders in view of their deployment for deriving one or more of lateralposition, velocity, gain or timing information while the data track theyare reading from may in one embodiment even contain no specific servopatterns at all but data only. Still, the main purpose of the servoreaders is to derive the subject information. In the same way, the onedata reader the servo readers are assigned to may in one embodiment evenread no data at all but servo patterns only in case the data track thedata reader is supposed to read from is completely written with servoinformation only. However, in typical embodiments this data track mayalso contain data written thereto.

In one embodiment each servo reader of the servo reader arrangement hasthe same width as a data reader, which width may be less but close tothe width of a data track. Each servo reader is assumed to be laterallyoffset from the one data reader by e.g., half of the data track width.In an ideal operational mode, the one data reader may be centered on anassigned data track and one servo reader of the servo reader arrangementis centered on the edge between the subject data track and an adjacentdata track. In this scenario, the readback signal of the one data readerexclusively contains a waveform stemming from data or servo patternswritten to the assigned data track while the readback signal of theservo reader shows a superposition of two waveforms, a first waveform ofwhich is the same waveform the data reader supplies in its readbacksignal given that the servo reader covers nearly half of this datatrack, and another waveform stemming from data or servo patterns in anadjacent data track which the servo reader also covers nearly half.Hence, the servo reader and the one data reader show the very samewaveform in their respective readback signals, however, the readbacksignal of the servo reader contains additional signal components fromthe adjacent data track. By means of correlating the readback signal ofthe servo reader with the readback signal of the one data reader, acorrelation between the two waveforms is obtained. By evaluating e.g.,an energy of the correlation signal at the output of the correlator, andby applying the knowledge about the offset between the servo reader andthe one data reader, a lateral position of the data reader and hence thetape head assembly relative to the assigned data track can bedetermined. In a different embodiment, the readback signal of the servoreader may be correlated with the known waveform representing the servopattern in the subject servo track. Preferably, the readback signal ofthe one data reader may also be correlated with the known waveform. Anevaluation of the energy of any of the correlation signal may be usedfor determining the current lateral position of the tape head assemblywith respect to the subject data track.

Whenever the tape head assembly leaves this ideal operational mode withthe one data reader being centered on the assigned data track, this datareader may become laterally deviated from its central position on theassigned data track. In the following, a sample deviation of a quarterof the data track width is assumed in a positive lateral direction intowhich the servo reader is arranged offset from this data reader. Hence,the servo reader is also off-centered from the edge between the datatrack and the adjacent data track by a quarter of the data track widthowed to the mechanical coupling of the servo reader and the one datareader. In this scenario, the one data reader covers the assigned datatrack still to nearly three quarters of its width, and nearly covers aquarter of the adjacent data track. The servo reader in turn coversnearly a quarter of the data track and nearly three quarters of theadjacent data track. Hence, when correlating the readback signals of theone data reader and the servo reader with each other or with the knownwaveform representing the servo pattern in the data track, there stillwill be a nonzero correlation signal, however, at a lower magnitudecompared to the centered position of the tape head assembly. Thecorrelation signal may again be used for determining the then newlateral position or the deviation from the centered positionrespectively. The known geometrical arrangement of the data reader andthe servo reader allows for reliably determining a maximum lateraldeviation of up to half the data track width.

In case the servo reader arrangement contains two servo readers whereina first one of the two servo readers is laterally offset by a positiveoffset value and a second one of the two servo readers is laterallyoffset by a negative offset value, a deviation of the one data readerand hence the tape head assembly from a lateral centered position ofmore than half of the data track width can be reliably detected, and inparticular a deviation up to a full data track width. Assuming the samedimensioning of the data reader, the servo reader and the data track asabove and having a second servo reader added which is laterally offsetfrom the one data reader by minus half of the data track width, adeviation of a maximum of up to plus/minus a data track width can bereliably detected. In this embodiment, each servo reader readback signalis correlated with the readback signal of the one data reader or withthe known waveform. By means of evaluating and comparing the energy ofthe two correlation signals obtained by correlating the readback signalof the data reader with the readback signal of each servo reader, thelateral deviation and its direction can be determined. A correlation ofthe readback signals of the two servo readers may allow for additionalinformation as to the lateral deviation and its direction.

In another embodiment, two servo reader arrangements are providedwherein the servo reader arrangements are assigned to a dedicated datareader each. Each servo reader arrangement preferably contains two servoreaders, wherein the servo readers of the one servo reader arrangementare offset from the one data reader by a positive and a negative offsetvalue respectively, and the servo readers of the other servo readerarrangement are offset from the other one data reader by a positive anda negative offset value respectively.

In view of the offset of the servo readers with one or more of the datareaders, the servo readers are arranged in at least one dedicated row inthe tape head assembly different to the one or more rows the datareaders of the set are arranged in. Different rows are meant to bearranged at different longitudinal positions in the tape head assembly,such that in one embodiment, all the data readers are arranged in acommon row while all the servo readers are arranged in a different rowlongitudinally offset from the data reader row.

As a result, the tape head assembly containing the set of data readers,the servo readers, and possibly the set of data writers no longer spansthe entire width of a data band between two servo bands (if any) butonly spans a portion of such width. This is advantageous since a lateralexpansion or contraction of the tape storage medium in response toenvironmental effects such as variations in temperature and/or humidityaffects the reading and/or writing from/to the tape storage medium lessthan in serpentine reading/recording with a tape head assembly with awider span. However, given that the tape head assembly no longer spansthe entire data band and may have access to servo informationprerecorded in the servo bands, the presently introduced concept of theservo reader arrangement is desired given that the tape head assemblymay in view of its low lateral span require to adjust its lateralposition without the aid of such servo bands even if these wereprovided.

As far as the aspect of the method for operating a tape storage systemis concerned, readback signals from the servo reader arrangement arereceived simultaneously to receiving the readback signals from the setof data readers. The servo reader arrangement contains one or more servoreaders arranged laterally offset from one of the data readers by lessthan the width of a data track. At least one of lateral position,velocity, gain and timing information is determined dependent on thereadback signals of each servo reader, and preferably in additiondependent on the readback signal of the one data reader. In anotherembodiment, there may be readback signals received from another servoreader arrangement assigned to another one of the data readerssimultaneously to receiving the readback signals from the set of datareaders. The other servo reader arrangement may contain at least oneservo reader being laterally offset from of the other one data reader byless than the width of a data track, and preferably two servo readersare laterally offset therefrom, one by a positive and one by a negativeoffset value. The at least one of lateral position, velocity, gain andtiming information may be determined dependent on the readback signalsof all the servo readers in the servo reader arrangements, andpreferably in addition dependent on the readback signals of the one andthe other one data reader.

For extracting the subject information from the various readback signalsa correlation is applied between the readback signal of each servoreader of a common servo reader arrangement and the readback signal ofthe assigned one data reader, or between each readback signal of theservo readers and possibly the one data reader with the known waveformeach. As indicated above, in case two of the readers out of the servoreaders and the one data reader at least partly cover the same datatrack, the associate readback signals at least contain the same waveformcomponent resulting in a nonzero correlation signal in the assignedcorrelator. The comparison of the energy of the auto-correlatedcomponents in the various correlation signals may allow for determiningthe lateral position, i.e., the deviation from the centered positionincluding a direction of the deviation.

The lateral position and/or deviation may finally be used forcontrolling an operation of the tape storage system during writingand/or reading data to/from the tape storage medium, e.g., forrepositioning the tape head assembly such that the data readers and/ordata writers are repositioned on their respective data track centers, orfor repositioning the tape head assembly at the end of the tape storagemedium for reading and/or writing from/to another data sub-band in areverse tape motion direction.

Given that the tape storage medium typically is a magnetic storagemedium, data and servo patterns if any in the data tracks arerepresented by magnetic transitions along the data tracks. Such magnetictransitions represent a waveform in the readback signal of a datareader. In this context it is emphasized, that the waveform shallencompass a representation of the subject signal in both, the timedomain and the frequency domain. In one embodiment, the data track theone data reader is supposed to read from may also or entirely containservo information in form of one or more different servo patternswritten to this data track. The waveforms of a servo pattern may in oneembodiment result from a predefined pseudo noise sequence written to thedata track. Specifically, a pseudo noise sequence comprises a sequenceof binary symbols and as such a defined bit pattern of a given length ofbinary symbols. In a different embodiment, a predefined waveformresulting from reading an underlying servo pattern may be characterizedin the spectral domain by a defined frequency of magnetic transitions inthe subject data track. Specifically, a waveform with a definedfrequency in the spectral domain is a periodic waveform in the timedomain, and preferably a sinusoidal waveform in the time domain.

In case of different servo patterns used the corresponding waveforms inthe readback signal may be orthogonal or quasi-orthogonal to each other.Orthogonality in adjacent track recording preferably is fulfilled if aproduct of the spectrum or a correlation of the time function of thesubject waveform with a waveform in an adjacent track is zero. Thismeans, that a correlation between any two different waveforms preferablyis zero or nearly zero as computed either in the time domain or in thespectral domain for all different waveforms applied, such that thewaveforms preferably used in the different data tracks, and hence theirunderlying magnetic transition schemes defining the different servopatterns, are preferably orthogonal or quasi-orthogonal. This featuremay be used in particular when servo patterns are written to adjacentdata tracks at the same longitudinal positions of these data tracks. Theservo readers of the servo reader arrangement may then, for example,also extract information by the correlation of the waveforms stemmingfrom the different servo patterns in the adjacent data tracks beingsuperimposed in the readback signal of a servo reader, and possibly theone data reader.

In case different bit patterns representing pseudo noise sequences areused as servo patterns, the orthogonality of waveforms resulting fromreading the different pseudo noise sequences is preferably verified inthe time domain. The readback signals of the data and/or servo readersthen are preferably analyzed in the time domain by means of correlationoperations. The different pseudo noise sequences used in one embodimentare preferably characterized by good cross-correlation properties. Suchsequences in one example may be Gold sequences. Alternatively, thepreferably at least two pseudo noise sequences used may originally bederived from a single pseudo noise sequence with good autocorrelationproperties. Such pseudo noise sequence may include Barker codes,complementary sequences or maximal length sequences. In this embodiment,the servo patterns of two adjacent data tracks are written with the samepseudo noise sequence, however, with a phase offset and/or oppositepolarities. An advantage of this scheme is that a detector may includeonly a single sequence correlator. In case different defined frequenciesof magnetic transitions are used as servo patterns, the orthogonality ofwaveforms resulting from reading the different frequencies of magnetictransitions is preferably verified in the spectral domain, by, e.g.,Discrete Fourier Transform operations.

For each of the above described variants, the underlying servo patternsmay preferably belong to a class of energy based servo (EBS) patterns.If such servo patterns of adjacent data tracks are read by a single dataand/or servo reader covering multiple of the data tracks while reading,the readback signal from the data and/or servo reader provides differentenergies of the different waveforms present in the readback signalsubject to the coverage of the different data tracks. By means ofestimating and/or evaluating the energies of the different waveforms inthe readback signal and specifically by building ratios between thevarious energies, additional servo information may be acquired. Any suchservo patterns used may aim at fast and accurate acquisition of lateralposition information. The servo patterns preferably have an inherenthigh density in magnetic transitions—this is why these servo patternshere also are referred to as high density patterns—in order to enableswift and accurate estimates of servo channel parameters such as thelateral position, thereby enabling improved tape head positioning at amuch wider tape velocity range and supporting for larger bandwidth headactuation.

In an embodiment, the tape storage medium to be read additionallycontains dedicated servo tracks with the set of data tracks beingarranged between these two servo tracks. The servo tracks mayexclusively be written and/or preformatted with servo patterns with nodata interspersed. The width of a servo track may correspond to thewidth of a data track. Preferably, the tape head assembly may include atleast one additional servo reader for reading from each servo track. Thededicated servo tracks may provide additional information such as timinginformation and/or other information such as LPOS (longitudinal positioninformation), servo band ID, servo track ID, manufacturing informationetc.

Servo patterns arranged in the data tracks, if any, are written togetherwith the data, i.e., written during the operation of the tape storagemedium in a tape drive which implies that the servo sections are writtenby the very same data writers that do write the data to the set of datatracks. The servo patterns of dedicated servo tracks, if any, maypreferably be preformatted servo patterns. Preformatting is typicallyexecuted by the tape manufacturer.

FIG. 1 schematically illustrates a tape storage system in a top view ona section of a tape TP that passes a tape head assembly THA. The tape TPhas a longitudinal extension in x direction and a lateral extension in ydirection. The longitudinal extension in x direction coincides with atape motion direction TMD, assuming there is no tape skew. Servo bandsSB and data bands DB extend along the longitudinal extension of the tapeTP. Each data band DB is arranged in between two servo bands SB. Eachdata band DB contains multiple sets of adjacent data tracks not shown indetail wherein preferably each set of adjacent data tracks of a databand DB is written and/or read simultaneously in case the tape headassembly THA comprises a number of data writers and data readersrespectively which number preferably corresponds to the number of datatracks in each set. A set of adjacent data tracks that is simultaneouslywritten and/or read is referred to as a data sub-band. Each servo bandSB contains servo patterns arranged along the longitudinal extension ofthe tape TP.

FIG. 2 illustrates the top view of a data sub-band, e.g., the datasub-band of FIG. 1 the tape head assembly THA currently covers. The datasub-band as shown may contain a set of e.g., sixteen adjacent datatracks DTj, j=[1 . . . 16]. The tape head assembly THA contains acorresponding number I of sixteen data readers DRi, i=[1 . . . 16] in aset for simultaneous reading from the sixteen data tracks DTj. Each datatrack DTj has a width wDT in y-direction. Each data reader DRi of thetape head assembly THA has a width wDR less than the width wDT of eachdata track DTj, and presently ˜0.8*wDT such that each data reader DRispans less than one data track DTj when being centered on a data trackDTj. The data readers DRi are arranged in a common row. Preferably, thedata readers DRi are embodied as magneto resistive (MR) elements.

The tape head assembly THA further contains two servo readerarrangements SRA1 and SRA2. Each servo reader arrangement SRA1, SRA2includes two servo readers SR1, SR2 and SR3, SR4 respectively. All theservo readers SRn are arranged in a common row in the tape head assemblyTHA which row is longitudinally offset from the row the data readers DRiare contained in. The servo reader SR1 has a positive offset (+ offset)with respect to the one data reader DR1 by plus half of the width wDT ofa data track while the servo reader SR2 has a negative offset (− offset)with respect to the one data reader DR1 by minus half the width wDT of adata track. The servo reader SR3 has a positive offset (+ offset) withrespect to the other one data reader DR9 by plus half of the width wDTof a data track while the servo reader SR4 has a negative offset (−offset) with respect to the other one data reader DR9 by minus half ofthe width wDT of a data track. Each servo reader SRn has a width wSRequal to the width wDR of a data reader, i.e., ˜0.8*wDT. The servoreaders SRn and the data readers DRi are arranged on separate layers ofthe tape head assembly THA manufactured in thin film technology.

The tape head assembly THA further contains a number K of sixteen datawriters DWk, k=[1 . . . 16], each with a width wDW exceeding the widthwDT of a data track DTj. The data writers DWk are arranged offset fromeach other in the longitudinal direction x, i.e., each data writer DWkis arranged at a different longitudinal position x on the tape headassembly THA. Given that the data writers DWk are designated to writeadjacent data tracks DTj that are arranged close to each other, suchstaggered arrangement leads to a reduced crosstalk during writing. Giventhat each data writer DWk has a width wDW exceeding the width wDT of adata track, data writer DWk assigned to write a data track DTj alsooverwrites a small portion of the adjacent data track DT(j+1). Thisapproach is referred to as shingled writing in order to enable writingdata tracks at small widths. The data readers DRi and the data writersDWk are mechanically coupled in the tape head assembly THA.

In one embodiment, all data tracks DTj of the set may solely containdata. In another embodiment such as shown in FIG. 2, at least the firstdata track DT1 may contain servo information encoded in servo patterns.This servo information may be factory preformatted. In this case, thepreformatted servo information may become overwritten when writing datatracks by means of the data writer DW1. In order to allow for a cleanoverwriting, a data eraser DE may be included in the tape head assemblyTHA. The data eraser DE may generate a clean edge adjacent to the datatrack DT1. Alternatively, some or all of the factory-preformatted servoinformation may be left unmodified when writing the tape. This approachmay avoid the accumulation of “written-in” noise in the servo signal andhence avoids degraded lateral and/or longitudinal sub-band placementwhen overwriting sub-bands many times, and may also simplify appendedwrite operations.

In an ideal operating mode as shown in FIG. 2, the data readers DRi arecentered on the data tracks DTj, while the servo readers SRn arecentered on edges between adjacent data tracks DTj. In this scenario theservo readers SR1 and SR2 extract information from data track DT1 and anadjacent track each, whereas the servo readers SR3 and SR4 extractinformation from data track DT9 and an adjacent track each. The datatracks DT1 and DT9 may include data and/or servo information, whichservo information may be provided by servo patterns representing pseudonoise sequences or sinusoidal waveforms, for example. Feedback for botha track- and skew-following mechanism may in one embodiment be obtainedby using the same data or servo patterns. Furthermore, some of the otherdata tracks DTj of the set may in another embodiment also includeinterspersed predefined i.e., “known” servo patterns to further enhanceservo signal extraction and/or for representing, e.g., LPOS and datasub-band ID information.

During an initial positioning of the tape head assembly, an initialcoarse positioning is followed by finding the desired data sub-bandwhich may be achieved by using some or all of the data readers DRi ofthe set to look for a servo pattern. Once a data sub-band ID isidentified, a distance of the current tape head assembly position from atarget position is estimated, and a control effort is generated to movethe tape head assembly THA towards the target position until the correcttape head assembly placement is achieved.

FIG. 3 illustrates the concept of extracting lateral positioninformation from a data and servo reader arrangement such as shown inFIG. 2 with respect to a single servo reader arrangement SRA1 includinga single servo reader SR1 only for illustration purposes. Diagram 3 a)shows the data track DT1 of the present data sub-band and a data trackDT16 of another data sub-band on top of the present data sub-band. Thedata reader DR1 is normally positioned on a center of the associate datatrack DT1. For illustration purposes, data reader DR1 is shown offsetfrom the center of the associated data track DT1 in diagram 3 a). Thedata track DT1 may include data and/or pseudo noise sequences serving asservo pattern. Similarly, the data track DT16 of the previous datasub-band may contain data and/or pseudo noise sequences, and may havebeen partially overwritten (shingled) by DT1 of the current datasub-band.

For illustration purposes, in diagram 3 b) the output signals of twocorrelators A and B are shown, wherein correlator A yielding signal WF1correlates the readback signals of the data reader DR1 reading a pseudonoise sequence PN_A written on track DT1 and a reference waveformrepresenting the pseudo noise sequence PN_A, and correlator B yieldingsignal WF2 correlates the readback signal of the servo reader SR1reading a pseudo noise sequence PN_B orthogonal to PN_A written on trackDT16 and a reference waveform representing the pseudo noise sequencePN_B. The positioning of the data reader DR1 and of the servo reader SR1is shown in diagram 3 a). Peaks in the correlator output signals WF1 andWF2 originate from the correlation of the data or servo patterns PN_Aand PN_B written on the data tracks DT1 and DT16 with the correspondingreference waveforms. The amplitude of the peaks is the same for the twocorrelator output signals, which indicates that the overlapping of thedata reader DR1 with the data track DT1 is the same as the overlappingof the servo reader SR1 with the data track DT16, as assumed in diagram3 a). In an embodiment, shown in diagram 3 c), the readback signal ofthe servo reader SR1 and the readback signal of the servo reader SR2 arecontinuously correlated by a correlator C. Note that the positioning ofthe data reader DR1 and of the servo readers SR1 and SR2 relative to thedata tracks DT1 and DT2 as illustrated in FIG. 2 would result in thelargest amplitude of the output signal of correlator C, whenever thesignals written in data tracks DT1, DT2, and DT16 have zero mean and areuncorrelated.

FIG. 4 shows a flow chart according to an embodiment of the presentinvention. In step S1, the method is started, for example, by activatingthe tape drive for reading data from a tape storage medium in a tapecartridge inserted into the tape drive. It is assumed that the tape isalready positioned at the desired lateral and longitudinal position.Then, while in step S1 the tape is already moved at a desired tapevelocity, in step S2 readback signals are simultaneously received fromthe servo readers of a servo reader arrangement and from the set of datareaders specifically including the one data reader the servo readers ofthe servo reader arrangement are offset from by less than the width of adata track. In step S3 the readback signals of the servo readers of thesame servo reader arrangement are correlated with the readback signal ofthe one data reader and are correlated with each other. In step S4lateral position information is derived from the result of the variouscorrelations and in step S5 the lateral position information is appliedto repositioning the tape storage medium to a reference position withrespect to the tape head assembly. The steps S3 to S5 are executed initerations until the end of the tape storage medium is reached, or untilthe tape transport is stopped, or until a global repositioning of thetape head assembly is performed (which actions are not included in theflow chart).

FIG. 5 illustrates a block diagram of a tape drive of a tape storagesystem according to an embodiment of the present invention, e.g., for atape storage system according to FIG. 2. The data readers DR1 and DR9 aswell as the servo readers SR1, SR2, SR3 and SR4 provide their readbacksignals to a control unit CU. The readback signals are received by acorrelation entity CE and a lateral position value is determined fromthese readback signals. The lateral position value is then convertedinto a position error signal by a determination unit PES. A tape headactuator controller TAC translates the position error signal PES into acontrol signal for the tape head actuator TA, which then adjusts thelateral position of the tape head assembly which comprises the data andservo readers.

It will be understood by those skilled in the art that manymodifications and permutations may be made without departing from thescope of the present invention.

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a method for operating a tape storagesystem, or a computer program product, a tape head assembly and a tapestorage system. Accordingly, aspects of the present invention, inparticular in form of the controller/control unit, may take the form ofan entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system.”Furthermore, aspects of the present invention, such as the method, maytake the form of a computer program product embodied in one or morecomputer readable medium(s) having computer readable program codeembodied thereon. Any combination of one or more computer readablemedium(s) may be utilized. The computer readable medium may be acomputer readable signal medium or a computer readable storage medium. Acomputer readable storage medium may be, for example, but not limitedto, an electronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system, apparatus, or device, or any suitable combinationof the foregoing. More specific examples (a non-exhaustive list) of thecomputer readable storage medium would include the following: anelectrical connection having one or more wires, a portable computerdiskette, a hard disk, a random access memory (RAM), a read-only memory(ROM), an erasable programmable read-only memory (EPROM or Flashmemory), an optical fiber, a portable compact disc read-only memory(CD-ROM), an optical storage device, a magnetic storage device, or anysuitable combination of the foregoing. In the context of this document,a computer readable storage medium may be any tangible medium that cancontain, or store a program for use by or in connection with aninstruction execution system, apparatus, or device.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer readable signal medium may be any computer readable medium thatis not a computer readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java, Smalltalk, C++ or the like and conventional proceduralprogramming languages, such as the “C” programming language or similarprogramming languages. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider). Computer program instructions may be provided to a processorof a general purpose computer, special purpose computer, or otherprogrammable data processing apparatus to produce a machine, such thatthe instructions, which execute via the processor of the computer orother programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer program instructions may also bestored in a computer readable medium that can direct a computer, otherprogrammable data processing apparatus, or other devices to function ina particular manner, such that the instructions stored in the computerreadable medium produce an article of manufacture including instructionswhich implement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

1. A method for operating a tape storage system, the method comprising: simultaneously receiving readback signals from a set of data readers reading from data tracks of a set of adjacent data tracks extending along a longitudinal extension of a tape storage medium wherein each data track of the set has a width and each data reader of the set has a width equal to or less than the width of a data track; simultaneously with receiving the readback signals from the set of data readers, receiving readback signals from a servo reader arrangement containing at least one servo reader arranged laterally offset from one of the data readers by less than the width of a data track; and determining at least one of lateral position, velocity, gain and timing information dependent on the readback signals of each servo reader in the servo reader arrangement.
 2. The method of claim 1, wherein the at least one of lateral position, velocity, gain and timing information is determined dependent on the readback signals of each servo reader in the servo reader arrangement and dependent on the readback signal of the one data reader.
 3. The method of claim 1, further comprising: receiving the readback signals from the servo reader arrangement containing a number of servo readers less than half the number of the data readers in the set, and preferably two servo readers a first one of which is laterally offset by a positive offset value from the one data reader and a second one of which is laterally offset by a negative offset value from the one data reader; and determining at least one of lateral position, velocity, gain and timing information dependent the readback signals of the two servo readers and the readback signal of the one data reader, by correlating the readback signals of the two servo readers with the readback signal of the one data reader.
 4. The method of claim 1, further comprising: simultaneously with receiving the readback signals from the set of data readers, receiving readback signals from another servo reader arrangement assigned to another one of the data readers, the other servo reader arrangement containing at least one servo reader being laterally offset from of the other one data reader by less than the width of a data track; and determining at least one of lateral position, velocity, gain and timing information dependent on the readback signals of each servo reader in the other servo reader arrangement.
 5. The method of claim 1, further comprising: receiving the readback signals from the other servo reader arrangement containing two servo readers a first one of which is laterally offset by a positive offset value and a second one of which is laterally offset by a negative offset value; and determining at least one of lateral position, velocity, gain and timing information dependent the readback signals of the two servo readers of the other servo arrangement and the readback signal of the other one data reader, and preferably by correlating the readback signals of the two servo readers of the other servo arrangement with the readback signal of the other one data reader.
 6. A non-transitory, computer readable storage medium having computer readable instructions stored thereon that, when executed by a computer, implement a method for operating a tape storage system, the method comprising: simultaneously receiving readback signals from a set of data readers reading from data tracks of a set of adjacent data tracks extending along a longitudinal extension of a tape storage medium wherein each data track of the set has a width and each data reader of the set has a width equal to or less than the width of a data track; simultaneously with receiving the readback signals from the set of data readers, receiving readback signals from a servo reader arrangement containing at least one servo reader arranged laterally offset from one of the data readers by less than the width of a data track; and determining at least one of lateral position, velocity, gain and timing information dependent on the readback signals of each servo reader in the servo reader arrangement.
 7. The storage medium of claim 6, wherein the at least one of lateral position, velocity, gain and timing information is determined dependent on the readback signals of each servo reader in the servo reader arrangement and dependent on the readback signal of the one data reader.
 8. The storage medium of claim 6, wherein the method further comprises: receiving the readback signals from the servo reader arrangement containing a number of servo readers less than half the number of the data readers in the set, and preferably two servo readers a first one of which is laterally offset by a positive offset value from the one data reader and a second one of which is laterally offset by a negative offset value from the one data reader; and determining at least one of lateral position, velocity, gain and timing information dependent the readback signals of the two servo readers and the readback signal of the one data reader, by correlating the readback signals of the two servo readers with the readback signal of the one data reader.
 9. The storage medium of claim 6, wherein the method further comprises: simultaneously with receiving the readback signals from the set of data readers, receiving readback signals from another servo reader arrangement assigned to another one of the data readers, the other servo reader arrangement containing at least one servo reader being laterally offset from of the other one data reader by less than the width of a data track; and determining at least one of lateral position, velocity, gain and timing information dependent on the readback signals of each servo reader in the other servo reader arrangement.
 10. The storage medium of claim 1, wherein the method further comprises: receiving the readback signals from the other servo reader arrangement containing two servo readers a first one of which is laterally offset by a positive offset value and a second one of which is laterally offset by a negative offset value; and determining at least one of lateral position, velocity, gain and timing information dependent the readback signals of the two servo readers of the other servo arrangement and the readback signal of the other one data reader, and preferably by correlating the readback signals of the two servo readers of the other servo arrangement with the readback signal of the other one data reader.
 11. A tape head assembly, comprising: a set of data readers configured to simultaneously read data from a set of adjacent data tracks of a tape storage medium, each data track having a width; wherein the width of each data reader of the set along a lateral extension of the tape head assembly is equal to or less than the width of a data track wherein the lateral extension of the tape head assembly is orthogonal to a longitudinal extension of the tape storage medium when arranged in a tape drive containing the tape head assembly spanning at least the set of data tracks during reading; and a servo reader arrangement containing at least one servo reader arranged laterally offset from one of the data readers by less than the width of a data track.
 12. The tape head assembly of claim 11, wherein: each servo reader of the servo reader arrangement has a width equal to the width of a data reader; and the number of servo readers is less than half the number of the data readers in the set.
 13. The tape head assembly of claim 11, wherein: each data reader has a width wDR=d*wDT, with wDT being the width of a data track, and 0.8≦d≦1; and each servo reader of the servo reader arrangement is offset from the one data reader by an offset=f*wDT, with wDT being the width of a data track, and 0.3≦|f|≦0.7.
 14. The tape head assembly of claim 11, wherein the servo reader arrangement contains two servo readers, wherein a first one of the two servo readers is laterally offset by a positive offset value and a second one of the two servo readers is laterally offset by a negative offset value.
 15. The tape head assembly of claim 11, wherein each servo reader of the servo reader arrangement is offset from the one data reader by an offset=f*wDT, with wDT being the width of a data track, and |f|=0.5.
 16. The tape head assembly of claim 11, wherein each servo reader of the servo reader arrangement has a width less than the width of a data track.
 17. The tape head assembly of claim 11, containing another servo reader arrangement containing at least one servo reader, and preferably two servo readers arranged laterally offset from another one of the data readers by less than the width of a data track.
 18. The tape head assembly of claim 11, wherein: the data readers of the set are arranged in at least one row in the tape head assembly; and the one or more servo readers are arranged in at least one different row in the tape head assembly at a different longitudinal position than the at least one row containing the data readers.
 19. A tape storage system comprising a tape head assembly according to claim 11 and a tape storage medium, the tape storage medium comprising at least one set of data tracks along a longitudinal extension of the tape storage medium and adjacent to each other, and at least one data track of the set comprising one of servo sections or data sections alternating with servo sections. 